Copyright © 2011 Open Geospatial Consortium,
Inc. All Rights Reserved.
17 combining multiple statements using the logical operators AND or OR. For instance in
the aviation use cases a common filter was: “Give me all events that occur during my flight time and thatare located within a buffer of 100 nautical miles around my flight
path.”
For the addressing of event properties the filter encoding recommends to use XPath if the event is encoded in XML. In the example above this would mean that the geometry
position in the event would be referenced by an XPath expression like “aixm:Airspacegml:boundedBy”. As an alternative one can define keywords
to refer to specific event properties like “geometry”. This can be simpler to implement, especially when complex properties like the geometry need to be accessed but the XPath
version is more flexible. In addition, the mapping of keywords to their XPath equivalent has to be document appropriately to avoid confusion. Note that the restrictions regarding
the XPath context described above also apply here.
6.3 OGC Event Pattern Markup Language
The Event Patter Markup Language EML is available as an OGC Discussion Paper OGC 08-132. It can be used to define rules for Complex Event Processing CEP and
Event Stream Processing ESP. It enables theinclusion of multiple events into the filtering process and even to derive new information.
EML was used, for example, in the OWS-7 SFE thread to detect when a vehicle entered a specific area. Instead of performing a rather simple and inexact spatial filter that just
detects location events to be within the area, it was checked if an event on the outside was followed by one on the inside. This way a vehicle entry event was derived and forwarded
instead of sending each position update from the inside.
Especially in situations when a forwarded event is used as a trigger for further processing by the receiver it can be important to reduce the event output to the necessary minimum.
Further discussions about the filtering of events can be found in the SES specification OGC 08-133, the OWS-7 Event Architecture ER OGC 10-060r1, and the OGC SWE
Service Model OGC 09-001.
6.4 Topics
Data at an Event Service is usually published and made available to subscription matching processes as one big cloud of data. New data is usually processed as soon as it
is available to the service. Topics as defined by WS-Topics provide a way to structure the data cloud. In a sense, they provide a specific view upon the data cloud. Views from the
database domain are a similar concept. In addition, TV channels also represent the concept. There, a Sports channel will publish Sports related news, while the news
published on a channel like CNN will have a more general scope.
In any case, this subsetting of the data cloud can help improve performance and ease the task of creating subscriptions. This is explained in more detail in section 6.3 of [OGC
